Problem: The scale on a map is 3cm : 4km. If the distance between two cities is 16km, how far apart in cm are the two cities on the map?
The scale means that every 3cm on the map corresponds to an actual distance of 4km. An actual distance of 16km is the same as 4 $\cdot$ 4km. The distance between the two cities on the map is 4 $\cdot$ 3cm, or 12cm.